A species-area relationship exists
for Anolis lizards on Caribbean islands. Using a phylogeny,
it is possible to identify whether a species has arisen in situ
within an island, or whether it arrived by colonization from another island.
The existence of sister taxa on the same island suggests the former possibility--for
example, the alutaceus group contains 14 species, all on Cuba.
The most parsimonious interpretation is that at least 13 speciation events
have occurred on Cuba, producing these species. By contrast, when
sister taxa occur on different islands, the most likely explanation is
that speciation occurred after populations became isolated on different
islands, either by colonization or vicariance.

Based on this approach, we investigated
whether a relationship exists between island area and rate of speciation.
We found, first, that a threshold island area exists below which speciation
rarely, if ever, occurs. This threshold falls between Guadeloupe,
which contains only 1 anole species, and Puerto Rico, which has 10.
Why speciation has not occurred on Guadeloupe, or other relatively large
islands, is not clear. The island is vegetationally and topographically
complex and anoles have inhabited the island for millions of years.
Second, among those islands which have experienced in situ speciation,
the rate of speciation increases with island area. This speciation-area
relationship is a potentially important complement to ecological explanations
for species-area relationships.